1. Field of Invention
The present invention relates to a method for producing high density materials with intrinsic, unabradable slipperiness and, more particularly, materials having low coefficients of friction and wear rates and containing hexagonal boron nitride.
2. Description of Prior Art
Friction, defined in the context of this invention as the resistance to movement of one solid body over another, is essential to the proper course of many physical, biological and industrial processes. In a vast number of situations however, it is disadvantageous, resulting in loss of power, generation of heat and wear of the surfaces in contact. Consequently, wherever friction constitutes a problem, the prior art has endeavored to lower the coefficient of friction of the surfaces in relative motion to one another.
In one approach, a film of fluid lubricant is interposed between the moving surfaces. While traditional fluid lubricants such as vegetable and mineral oils, greases and animal fats still hold their place in many modern industrial applications, they do not possess the properties necessary to effectively function as engine lubricants, transmission fluids, gear oils, hydraulic and metalworking fluids and other high performance lubricants. Also, new high performance lubricant formulations are constantly required to overcome the shortcomings of existing products and to meet the demands of advancing technology. A significant example is the development of inorganic solid lubricants for applications involving severe temperatures, pressures and environments which would preclude the use of organic fluids.
Solid lubrication is based on ductile shear within a solid lubricant film caught between moving surfaces. Consequently, solid lubricants must have low shear strength, a property they share with fluid lubricants. The most widely used solid lubricants, graphite and molybdenum disulfide, as well as a number of other dichalcogenides provide this property through slip along preferred crystallographic planes. Solid lubricants should also have low abrasivity, i.e. they should be softer than the surfaces they lubricate to avoid causing abrasive wear to the latter. Finally, they should be thermodynamically and chemically stable in the environment in which they are to function. Yet, all of the solid lubricants mentioned so far suffer thermal decomposition or oxidative degradation at temperatures above 500-600° C. In addition, graphite is electrically conductive and reactive with ferrous alloys. To transcend these limitations, the prior art has turned to hexagonal boron nitride (h-BN).
Hexagonal boron nitride (h-BN) powder is synthesized by nitridation or ammonolysis of boron trioxide at temperatures ranging from 800° C. to 2000° C. During h-BN powder synthesis, edge-fused six-atom (B—N)3 hexagonal rings crystallize into two-dimensionally-stacked layers of h-BN macromolecules. Different crystalline structures can be obtained by changing process conditions. During the initial stage of crystallization, from 800° C. to about 1200° C., h-BN macromolecules grow into hexagonal primary crystallites of approximately 10 nm width termed turbostratic boron nitride (t-BN). During the next step of the crystallization process, from about 1200° C. to about 1800° C., the crystallites coalesce, roughly tripling their size to about 30 nm into so-called mesographitic boron nitride (m-BN). During the final stage of crystallization, from about 1800° C. to 2000° C. the crystallites again roughly triple their size to about 90 nm, arranging themselves into regularly stacked layers with the B atoms in the rings in one layer above and below N atoms in the rings of contiguous layers in what is called graphitic or fully crystallized hexagonal boron nitride (h-BN). It will be noted that the term hexagonal boron nitride (h-BN) is ambiguous as it is used to designate both the generic hexagonal crystalline structure of boron nitride—as distinct from other crystalline structures such as cubic boron nitride (c-BN)—as well as the fully crystallized graphitic boron nitride.
In the stacked layer arrangement of hexagonal boron nitride (h-BN), the polar interplanar B—N bond between adjacent layers is longer and therefore weaker than the strong covalent intraplanar B—N bonds in the fused six-atom rings. As a result, h-BN crystal grains are easily cleft into flakes liable to slip relative to one another, hence exhibiting solid lubricity. Hexagonal boron nitride (h-BN) is useful as both a low temperature and high-temperature solid lubricant or in applications where the electrical conductivity or chemical reactivity of graphite would constitute a problem. Like other solid lubricants, h-BN films are commonly applied by burnishing, thermal spraying, electrodeposition, chemical or physical vapor deposition.
Hexagonal boron nitride powders have also found use in fluid film lubrication. Denton et al., U.S. Pat. No. 5,589,443 teaches an h-BN-filled grease composition for lubricating journal bearings in rock bits used for oil well drilling. Watari et al., U.S. Pat. No. 5,985,802 discloses a high-performance cutting or grinding oil containing a dispersion of fine h-BN powders.
In another use of h-BN toward reducing the coefficient of friction, the prior art has resorted to various coatings and treatments of the surfaces in relative motion to one another. In one such instance, Brown, U.S. Pat. No. 6,576,698 teaches surface coatings for firearm projectiles and firearm components such as gun barrels composed of thermosetting resins filled with fine h-BN particles.
In cases where neither solid lubricant films nor surface treatments are indicated to lower the coefficient of friction of surfaces in relative motion to one another, the prior art has endeavored to replace the system components displaying excessive friction with components made from materials having an intrinsic lower coefficient of friction. For example, in replacement of the lead-containing aluminum and copper alloys used by the prior art for journal bearings and bushings and other articles subject to high bearing loads, Dunmead et al., U.S. Pat. No. 6,837,915, (Dunmead) teaches hot isostatically processed, lead-free, high theoretical density, h-BN-containing metal alloys having low coefficients of friction and wear rates. Dunmead claims that the amount of h-BN needed to result in a reduction of at least 20% of the coefficient of friction concomitant with a reduction of not more than 10% of the yield strength of the boron nitride-free matrix material is 2-5 weight percent. Metals amenable to Dunmead's invention include iron, carbon steel, stainless steel, chromium, aluminum, copper, brass, bronze and other copper based alloys. While Dunmead's invention constitutes a major improvement over the prior art in providing high density, wear-resistant materials with low coefficients of friction—thereby obviating the recourse to circuitous techniques such as fluid lubrication or surface treatments—materials amenable to his invention are limited to hot isostatically processable metallic alloys. Also, hot isostatic pressing, being a cost-intensive process, is generally uneconomical for mass production and frequently necessitates secondary machining if the shapes of the end products are somewhat complex.
Consequently, the prior art has turned to pressureless sintering. However, attempts at pressureless sintering of either pure h-BN or h-BN dispersed in a matrix have been largely unsuccessful due to boron nitride's reluctance to sinter as a result of the strong covalent bonds on the crystal basal planes.
For instance, Hagio et al., in the above cited reference, describes pressureless sintering of cold-pressed compacts of pure h-BN powders at 2000° C. for 1 hour without achieving any densification at all.
Likewise, Pickens et al., in the above cited reference, reports on pressureless sintering of cold-pressed structures composed of alternating layers of silicon nitride and boron nitride at 1800° C. for 3 hours resulting in poor densification with ensuing inferior mechanical properties.
Ciloglu et al., U.S. Pat. No. 4,927,461 (Ciloglu) discloses pressureless sintering of ferrous powder metallurgy (P/M) compacts containing h-BN powder. However, Ciloglu only aims at improving machinability and, therefore, the amount of h-BN powder added to the ferrous matrix is infinitesimal, from 0.01 to 0.5 weight percent, i.e. far below Dunmead's aforementioned optimum 2-5 weight percent necessary to reduce the coefficient of friction by at least 20%. Significantly, Ciloglu's matrix is a typical ferrous P/M powder with a maximum particle size of 300 μm.
Braillard et al., U.S. Pat. No. 7,128,962 (Braillard) claims pressureless partial sintering of materials containing 10-40 volume percent of boron nitride agglomerates having an equivalent diameter of 5-200 μm with, the explicit intention of rendering the resulting matrix composite abradable. Braillard emphatically excludes any alloys containing nickel on the grounds of severe incompatibility between nickel and boron. This inherently eliminates a large number of stainless steels and superalloys from the reach of his invention. Significantly, Braillard's metal matrix alloy powder has a particle size range of 10-70 μm.
Cherubini et al., (Cherubini) in the above cited reference, reports on attempts to pressureless sinter cold-pressed compacts of minus 100 mesh (149 μm) type 304 stainless steel powder blended with minus 325 mesh (44 μm) boron nitride powder resulting in precipitation of a boron-rich phase throughout the stainless steel matrix.
Chou, T. C., (Chou) in the above cited reference, reports interfacial reactions during the chemical interdiffusion between boron nitride and Ni3Al with concomitant formation of Kirkendall voids.
Benko, E., (Benko) reporting in the above cited reference on the study of BN-metal systems, determines that the metals react with boron nitride forming borides or mixtures of borides and nitrides.
In the course of the development toward the present invention, attempts at pressureless sintering of h-BN containing matrix composites under conditions of temperature, pressure and atmosphere commonly used to sinter the corresponding boron nitride-free matrix material invariably failed, resulting in segregation of boron-rich precipitates. For example, the usual sintering temperature for boron nitride-free 304L stainless steel is about 1350° C. but when attempting to sinter 304L matrix composites containing 5 weight percent of h-BN at that temperature, liquid phase exuded from the matrix.
From the foregoing description of the prior art, it can be gathered that the ability to extend the reach of Dunmead's invention to a wider range of materials, that would encompass pure metals, metal alloys, oxides, nitrides, carbides, including cemented carbides, and mixtures of these, as well as the ability to reach the high densities essential to ensure wear resistance by pressureless sintering would be desirable and innovative improvements. However, the fabrication by pressureless sintering of highly densified boron nitride containing materials having low coefficients of friction and wear rates is not known.